1. Field of the Invention
The present invention relates to image processing apparatus, and more particularly to an image processing apparatus employing an error diffusion type binarizing system.
2. Description of the Related Art
In the field of image processing apparatus, an error diffusion system is known as a binarizing system that allows coexistence of gradation and resolution. Many systems have been proposed to improve the coexistence of gradation and resolution even in a mixed text/picture image by combining a region determination process and the error diffusion system.
One such proposal, for example, is disclosed in Japanese Patent Laying-Open No. 63-214073. An error calculation unit of an image processing apparatus disclosed in the Laid-Open application is shown in FIG. 6.
Referring to FIG. 6, multivalue input data is added with a binary error of a peripheral pixel and then binarized by a comparator 31. If the error added data is equal to or greater than a binary threshold value, "1" is provided as a binary output value. If the error added value is smaller than the binary threshold value, "0" is provided. By this process, binary error data of a target pixel is propagated to subsequent pixels, whereby a density retaining type binarization is carried out. In FIG. 6, a multiplier is provided within the feedback loop to apply a gain operation on a calculated error depending upon the feature of an image. For example, the gain is set as gain=0 for an image of text, gain=0.5 for an image of mixed text and pictures, and gain=1 for an image of a picture.
The conventional image processing apparatus shown in FIG. 6 had a problem that the propagation of a binary error is increased since a binary error calculation base value is fixed independent of the density of the image of an original.
Furthermore, it is difficult to speed up the operation of an error diffusion system employed in a conventional image processing apparatus. This is because an error diffusion system is implemented with asynchronous circuitry including a feedback loop. An error diffusion circuit has its operation speed limited according to the delay time of this feedback loop. Because an adaptation process for region determination is provided in the feedback loop as shown in FIG. 6, the operation speed is reduced in carrying out a process with high accuracy. In contrast, if the operation speed is increased, the accuracy is reduced.